1. Field of the Invention
The present invention relates to a screw for use in extruders and, particularly, to the structure of a screw for reducing the pulsation of an extruded product and a production process therefor.
2. Description of the Prior Art
In the prior art, to manufacture a rubber member which requires a predetermined sectional form, such as a tire tread or a side wall, an extruder comprising a screw is generally used. For instance, in the step of extruding a tire tread, a kneaded and blended rubber material for treads is heated with a warming mill to be softened, supplied to the hopper 51 of an extruder 50 shown in FIG. 3, carried to the end of a cylinder 53 and extruded from the nozzle 55 of a head 54 attached to the end of the cylinder 53 by a screw 52 to be molded into a predetermined sectional form, cooled and cut to a required length.
However, as the above extruder 50 generally has a short length (L/D), the shape of the rubber material to be molded is easily affected by the revolution of the screw 52. Therefore, when the extrusion rate (delivery rate) is increased by changing the structure of the screw, pulsation caused by delivery becomes large.
For example, when a high-delivery type screw 62 having a double-thread structure as a whole and a single-thread structure only on the hopper port 51s side is used as shown in FIG. 4, the delivery rate becomes higher than in the prior art. However, as all the rubber material supplied by the above single-thread structure portion is forced into the cylinder 53, a change in pressure applied from the rubber material to the flight portion 62F of the screw 62 become large in the vicinity of the hopper port 51s. Therefore, the pulsation of the delivered extruded product A becomes large, thereby increasing a gauge fluctuation.
Particularly when the extruded product is a tread, the above gauge fluctuation exerts a bad influence upon the uniformity and balance of tires. Therefore, the development of a screw capable of suppressing pulsation even when the extrusion rate is increased is desired.
It is an object of the present invention which has been made in view of the above problem of the prior art to provide a screw for extruders capable of reducing the pulsation of an extruded product while a high delivery rate is maintained.